Historically, the calculation of divergences in the fourth-derivative
QG started in the work [1]}, using Feynman diagrams
and separating massless and massive internal lines. This way to
make calculations was never used after that (up to our knowledge)
as all subsequent calculations were performed by technically more
simple heat-kernel (or Schwinger-DeWitt) technique, adapted for
the four-derivative operators in [2]} and
later on, in a more systematic way, in [3]}. However, the
diagrammatic approach of the pioneering work of Julve and Tonin
[1]} may be, by the end of the day, rather important. The
reason is that, using diagrams enable one to separate the graphs
with only massless internal lines from the ones which have the
lines of the massive degrees of freedom (i.e., ghosts, in the present
case). This method can be used to explore
the IR limit of the quantum corrections in higher derivative theories.
As we already mentioned above, in this area we have only the general
statement about the universality of quantum GR as an effective theory
of QG in the IR [2]}, [6]} and the calculations of decoupling
in the models of massive matter fields [7]}, [8]} (see further
references therein) that can be used as toy models for the problem
of decoupling massive degrees of freedom in QG. Let is stress that
completing the program of decoupling in the higher derivative models
would be important for improved understanding of QG as a whole.
